Geology and mining practice in relation to shallow subsidence in the Northern Coalfield, New South Wales

The former Northern Coalfield, comprising much of the Newcastle-Maitland-Ce ssnock-Lake Macquarie region, has now been mined for two hundred years. Mos t of the twenty-odd seams in the Newcastle, Tomago and Greta Coal Measures have been worked, but the bulk of production has been drawn from just four coalbeds: the Borehole, Greta, Homeville and Great Northern Seams. Literall y hundreds of mine names are on record for the coalfield, the great majorit y working these seams at depths of less than 30 m. This paper reviews regio nal subsidence problems in the context of mining history, past mining pract ices and coal measures geology. Subsidence over shallow bord-and-pillar wor kings may result in highly disturbed ground, open fissures, cratering. seam fires and 'creeps' (slow surface movements caused by gradual closure of mi ne cavities). This form of subsidence differs from that over modern longwal l mines in being less predictable, affecting much smaller areas, and occurr ing sometimes decades after mine abandonment. The geological factors contri buting to this subsidence hazard include the multiplicity of seams, the str ong roof sandstones, stiff coal and relatively soft floors, and the presenc e of faults and dykes. The working of thick seams like the Borehole Coal in up to three phases, leaving high, slender pillars, also contributed to lat er ground instability. Contrary to expectation, the main problem areas are not over haphazardly laid-out convict-era pits, but rather above small hand -worked scavenger mines ('ratholes') which operated as late as the 1950s. M any of these short-lived collieries still contain large voids at shallow de pth, sometimes only a few metres, which must eventually collapse or be back filled.